The conventional fuel vehicle adopts an engine as the power output source, when it is in downhill mode and its gear position is at forward gear, due to the speed control of the engine, if the vehicle speed is too high, the engine may automatically reduce the quantity of fuel injected, and the mechanical friction may generate a reverse towing and braking torque, so the vehicle speed will not be too fast. And most of the electric vehicles adopt the torque controlled motor as the power output source, and the motor controller may calculate the output torque of the motor according to the equation T=T2×Gain, and control the motor to drive the wheels of the vehicle by using the torque T, in order to ensure the vehicle reachs the required speed. Wherein T is the output torque of the motor, T2 is the maximum output torque of the motor under the current vehicle speed of the vehicle, Gain is the accelerator-pedal travel value which is in the range of 0%˜100%, and the value of Gain is proportional to the step depth of the accelerator pedal. When the vehicle is in downhill mode, T2 will be increased with the increasing of the vehicle speed, even if Gain is constant, the output torque T may also be increased. When its speed is too high, the friction of motor is inadequate to form the reverse brake, it may cause the vehicle speed too high, and then the vehicle is out-of-control, and serious accidents caused.